1. Field of the Invention
The present invention relates to high peak power ultrafast semiconductor lasers. In particular, the invention relates to a device and method for generating sub-nanosecond high peak power single pulses without respective emission tails in response to applying electrical bipolar pulses to the semiconductor laser diode.
2. Prior Art
A laser (acronym for light amplification by stimulated emission of radiation) is an optical source that emits photons, i.e. light radiation, in a coherent beam. Many materials have been found to have the required characteristics to form the laser gain medium needed to power a laser, and these have led to many types of lasers with different characteristics suitable for different applications including, but not limited to, science, the defense industry, medicine, and consumer electronics. The present disclosure is particularly concerned with semiconductor lasers.
As illustrated in FIG. 1, the simplest semiconductor laser 10, sometimes called a diode laser, comprises a single junction 12 between n- and p-type conductors 14 and 16, respectively. As electrons and holes are injected across junction 12 upon applying a current signal I, they form a gain active region 22 operative to transfer external energy into a laser beam 20 emitted from one of its opposite mirrored facets 18. The gain active region 22 is, thus, a material of controlled purity, size, dopant concentration, and shape, which amplifies the beam by the quantum mechanical process of stimulated emission.
The stimulated emission is the process by which, when perturbed by a photon, matter may lose energy resulting in the creation of another photon with the substantially same phase, frequency, polarization, and direction of travel as the original photon. In a semiconductor laser, the injected carriers—electrons—are absorbed by the laser medium, placing some of its particles into high-energy (“excited”) quantum states. The term “absorption” refers to the process in which the energy of the injected carriers is transferred to an atom whose valence (low energy) electrons make transition between two electronic energy levels. The absorbed energy may be re-emitted as radian energy. As pumping continues, the carrier (electron) density within an active gain region may be increased from below to above a lasing threshold—the lowest excitation level at which the laser's output starts building up due to coherent stimulated emission. At the threshold, the number of particles in one excited state starts exceeding the number of particles in some lower-energy state—the phenomenon known as population inversion. Further pumping leads to a generation of additional exited particles. Exited particles tend to return to a lower-energy state(s) while releasing respective photons. The latter, in turn, collide with injected carriers thereby emitting more photons defining the optical output power of the pumped laser.
The output light beam may be a constant-amplitude output (continuous wave); or a pulsed output which is of a particular interest here and achieved by using Q-switching, modelocking, gain-switching or other known techniques each allowing a laser to output high peak power (intensive) pulses. Gain-switching is the simplest technique since neither external cavity nor sophisticated fabrication technology is required for producing intensive optical pulses in the picosecond range (10−12 s).
Gain switching relies upon the switching of the optical gain through the diode laser pump current modulation using special driving circuit and can be realized buy using laser diodes of any structure. This technique includes exciting the first spike of relaxation oscillation and terminating the electrical pulse before the onset of the next spikes.
FIGS. 2A-2C illustrate the operation of the gain-switched diode and certain disadvantages associated with this technique. Specifically, FIG. 2A illustrates an injection current pulse 15 (FIG. 2A) is applied to a laser diode. In response, the carrier density n (FIG. 2B) reaches a threshold density no at time to causing the net gain to became positive and start the lasing process. As a consequence, the photon density P (FIG. 2C) starts rapidly increasing from spontaneous noise level to beyond a saturation level Pi where the net gain starts decreasing due to stimulated emission. The photon density P continues to increase above Pi level while the net gain is positive. At the same time, the population inversion decreases through stimulated emission which eventually leads to the negative net gain. At this point the photon density reaches its maximum Pmax and starts decreasing due to the negative net gain and at the same time continue to decrease population inversion. Subsequently, carrier density n drops below its threshold no which, theoretically should cause the termination of the laser pulse. Note that in order to restrict the optical emission to one single pulse, current pulse 15 should be switched off before the termination of optical pulse 26. However, even in this case because of a substantial population of the quantum well by carriers accumulated earlier in the active gain region, secondary oscillations or an emission tail 30 (FIG. 2C) of the optical output are typically observed.
Numerous methods including, but not limited to, an artificially induced saturable absorber and a spectral filter have been studied and widely disclosed in the past. However, the implementation of these methods in the context of the gain-switched mode of operation of a laser diode typically does not efficiently suppress the emission tail. The physics of emission tail 30 or, rather, the detrimental presence of the free carriers remaining after the termination of the first optical pulse is explained in, for example, a paper entitled “High power gain-switched laser diode . . . ”, which is published in Applied Physics Letters, 89, 081122 (2006) and fully incorporated herein by reference. Overall, the secondary or oscillatory optical pulses or emission tail 30 are undesirable in applications of laser diodes requiring high peak power optical pulses in the sub-nanosecond range.
A need, therefore, exists for a method of controlling high peak power gain-switched diode so that the above-noted problems encountered by the known prior art are minimized.
Another need exists for a high-peak power gain-switched laser diode operative to generate a picosecond-range (ps) high peak power pulse without or with a substantially suppressed emission tail.
Still another need exists for a semiconductor-based module or device including a driver and a gain-switched laser diode, which generates single picosecond-range intense optical pulses each with a substantially suppressed emission tail.
A further need exists for a semiconductor-based module or device configured with a driver, which is operative to generate a bipolar current pulse, and a semiconductor-based laser diode, which is coupled to the driver and operative to generate intense picosecond-range optical pulses each exhibiting a substantially suppressed emission tail in response to the bipolar electrical pulse.